The invention relates to a fixation system for bones with a connection carrier and at least one bone screw or at least one locking bolt.
Such fixation systems are used in osteosynthesis, wherein the bone screws are connected to the ends of the bone and the connection carrier bridges the fracture. The connection carrier may in particular be a bone plate, a marrowbone nail or a fixator. With this it is desirable, whilst adapting to the nature of the bone part to be connected, for the optical alignment onto the fragments or for compensating target errors, to be able to incorporate the bone screws at different angles into the connection carrier.
For this with a known fixation system the bone screws have heads with a roughly hemispherical-shaped seat surface of which one seat surface in passage holes is allocated to a bone plate. If for example with a tibia fracture the two bone pieces must be connected to one another, the metallic bone plate is applied onto the set-up bone pieces. Thereafter the screws are rotated into the bones such that the seat surfaces of the screw heads and of the plate holes come to bear on one another and the plate is pressed against the bone. From this there results a connection of bone parts, bone plate and bone screws. It however has been shown that a loosening of the connection of bone screws and bone plates may take place. A cause lies in the insufficient stability of the angle connections of bone screw and bone plate which are secured by friction forces between the screw head and the plate hole.
An angular stable connection of the bone screw and bone plate on the other hand leads to a gain in stability of the whole assembly. Various solutions are known in order to achieve such a stable combination. According to EP-A-10 201 024 this may for example be effected in that to the bone plate there is allocated a pressure plate which can be tensioned with the screw heads and fixed to this in a selected angular position. Such complicated fixation systems with a pressure plate are on account of their relatively large volume restricted in their usability.
Another solution according to WO/89 041 50 exists in widening the screw head in a slot region with a spreading screw and by way of this to press it into the plate hole. With this the screw head or the insert surrounding this as well as the plate hole have spherical seat surfaces which permit an alignment at various angles. This fixation system is likewise complicated in the manufacture and application.
Furthermore it is also known to provide the screw head with an outer thread and the plate hole with an inner thread. If then the screw is rotated into the bone then by way of a thread connection there is effected an angularly stable alignment of the plate and screw. This solution has however the grave disadvantage that the screw may not be mounted into the plate hole at any angle but only in the alignment predetermined by the thread axes.
This disadvantage according to DE 43 43 117 A1 is overcome in a manner that the means for fastening the bone screw at a certain angle to the bone plate comprise a threaded connection of the seat surfaces of the bone plate and the bone screw, this thread being formed by rotating in the bone screw at a certain angle by a preformed thread on at least one seat surface. With this the seat surface of the bone screw as well as also the seat surface of the bone plate may comprise a preformed thread. On rotating in the bone screw by way of the at least one thread there arises a deformation of the material in the seat surfaces and thus a formation of a thread connection in the respective screwing direction.
This solution has however in particular with the use of thicker plates the considerable disadvantage that the force for deforming the material is considerable and this intraoperationally may not always be mustered without any problem. In order to make this possible therefore in DE 196 29 011 A1 there is specified an additional bolt-shaped performer for aligning in bone bores with an outer thread for deforming a thread in the passage hole of a connection carrier. This deforming of a thread however has the disadvantage that it requires an additional working procedure and leads to the occurrence of wear and swarf particles from the wall region of the passage hole. Examinations have shown that with the application of pure titanium in the passage hole of a tibia plate there occurs a swarf formation of 0.0001 g per passage hole. A similar quantity also results from conventional metal plates. This is to be seen as disadvantageous for reasons of compatibility as well as due to the fact that here it is case of a foreign body.